In the simplest form, an organic electroluminescent (EL) device is comprised of an organic electroluminescent media disposed between first and second electrodes serving as an anode for hole injection and a cathode for electron injection. The organic electroluminescent media supports recombination of holes and electrons that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. A basic organic EL element is described in U.S. Pat. No. 4,356,429. In order to construct a pixelated OLED display that is useful as a display such as, for example, a television, computer monitor, cell phone display, or digital camera display, individual organic EL elements can be arranged as pixels in a matrix pattern. These pixels can all be made to emit the same color, thereby producing a monochromatic display, or they can be made to produce multiple colors such as a three-pixel red, green, blue (RGB) display. For purposes of this disclosure, a pixel is considered the smallest individual unit, which can be independently stimulated to produce light. As such, the red pixel, the green pixel, and the blue pixel are considered as three distinct pixels.
Color OLED displays have also recently been described that are constructed as to have four different colored pixels. One type of OLED display having four different colored pixels that are red, green, blue, and white in color is known as an RGBW design. Examples of such four pixel displays are shown in U.S. Pat. No. 6,771,028, U.S. Patent Application Publication Nos. 2002/0186214 A1; 2004/0113875 A1; and 2004/0201558 A1. Such RGBW displays can be constructed using a white organic EL emitting layer with red, green, and blue color filters for the red, green, and blue pixels, respectively. The white pixel area is left unfiltered. This design has the advantage that the organic electroluminescent media does not require patterning between the different colored pixels, thereby simplifying the manufacturing process. Furthermore, inclusion of the unfiltered white pixel allows for the display of most colors at reduced power consumption compared to similar RGB displays having a white organic EL emitting layer with red, green, and blue filters for the red, green, and blue pixels, respectively.
OLED displays driven with active matrix circuitry have also been shown. Active matrix circuitry typically includes active circuit components such as multiple transistors and one or more capacitors per pixel as well as signal lines such as data, scan, and power lines, which are shared by the pixels of a row or column. Each pixel in an active matrix OLED display is provided with at least one power transistor. A power transistor regulates the current flow to the pixel's organic EL element in response to a data signal provided on a data line. The power transistor draws current from a power line, which is electrically connected to a voltage source. This current is passed to the first electrode and the organic EL media of the pixel's organic EL element. The second electrode, which is disposed above the organic EL media and the active matrix circuitry, is then electrically connected to a second voltage source, which completes the current path. Examples of organic EL displays driven by active matrix circuitry are shown in U.S. Pat. Nos. 5,550,066; 6,281,634; and 6,456,013.
However, in OLED displays driven by active matrix circuitry, the voltage difference between the voltage source electrically connected to the power line and the voltage source electrically connected to the second electrode is at a level sufficient to power all the pixels connected to the power line at their highest level of intensity. This voltage difference, or drop, is split between the organic EL element and the power transistor. Therefore, when pixels are operated at a lower level of intensity, the supplied voltage is more than is required and the percentage of the voltage drop across the power transistors in these pixels is increased. Since power consumption is a function of the current flow and the voltage drop, this extra voltage drop results in poor power efficiency.
Active matrix OLED displays have been shown where different colored pixels are connected to the same power line, e.g. U.S. Pat. No. 6,456,013. Also, active matrix OLED displays have been shown where adjacent columns of pixels share the same power line in U.S. Pat. No. 6,522,079. Similarly, examples where the same power line is shared by pixels of differing colors or pixels in adjacent rows for an RGBW type active matrix display are shown in U.S. Pat. No. 6,771,028. However, such different colored pixels frequently require different maximum voltage levels. In such displays, the voltage is commonly set at a level to drive the most demanding pixels, resulting in poor power efficiency.